1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a power transmission belt having an improved load-carrying section that accounts for a dimensionally stable belt which elongates minimally during operation and has a high power transmission capability.
2. Background Art
The load-carrying section of a power transmission belt significantly affects the transmission capabilities of the belt, the flex fatigue, dimensional stability over time, as well as other critical belt properties.
Designers of power transmission belts have, over the years, dealt with two competing objectives--that of maintaining dimensional stability over the life of the belt and minimizing belt elongation during operation By increasing the dimensional stability of the belt over its life, the problem of belt elongation is aggravated. Conversely, alleviating the problem of belt elongation produces a dimensionally unstable belt.
In dealing with the above two objectives, belt designers have concentrated on the load-carrying section and, more commonly, on one or more longitudinal cords in the load-carrying section It is known in the prior art to fabricate a power transmission belt cord from a high strength polyester fiber Polyester fibers exhibit excellent mechanical and thermal properties that make them particularly suitable for the belt environment. In constructing cords with the polyester fibers, typically a plurality of the fibers are twisted about themselves into yarn bundles, which are in turn twisted into an untreated cord. The cord is subjected to an adhesive treatment and subsequent stretching thermal fixation treatment to produce a treated cord which is then integrated into a rubber layer under a predetermined tension and vulcanized to produce a load-carrying section.
Shrinkage percentage tends to rise as the shrinkage stress of the treated and dry heated cord is increased during the stretching thermal fixation treatment. While the stretching thermal fixation treatment of cords at high temperatures minimizes elongation of the belt during operation, it also reduces the dimensional stability of the belt over time. Heretofore, known manufacturing techniques for power transmission belts have been generally ineffective in making load-carrying section cords and belts therefrom with a high thermal shrinkage stress and a small percentage of dimensional shrinkage during the belt life.
Several attempts have been made in the prior art to overcome the above problems. For example, in Japanese Patent Examined Publication No. 50578/1980, a method of belt manufacture is disclosed in which a polyester fiber cord is thermally stretched through multiple stretching steps at a stretching ratio of at least 5%. The transmission efficiency is increased by thermal shrinkage due to frictional heat generated by belt slippage during operation.
Japanese Laid-Open Publication No. 231044/1985 discloses a method of improving the resistance to flex fatigue and dimensional stability of a belt by constructing the load-carrying section using rapidly spun polyester filament. The filament has at least 85 mol % content ethylene terephthalate as the repeating unit with an ultimate viscosity of at least 0.8%, less than a 0.19% birefringence rate, less than 60 degrees of orientation of the amorphous portion and the terminal carboxyl group content is less than 15 equivalence/10.sup.6 g.